Problem: A circle has a circumference of $20\pi$. It has an arc of length $4\pi$. What is the central angle of the arc, in radians? ${20\pi}$ ${\dfrac{2}{5}\pi}$ $\color{#DF0030}{4\pi}$
Explanation: The ratio between the arc's central angle $\theta$ and $2 \pi$ radians is equal to the the ratio between the arc length $s$ and the circle's circumference $c$ $\dfrac{\theta}{2 \pi} = \dfrac{s}{c}$ $\dfrac{\theta}{2 \pi} = 4\pi \div 20\pi$ $\dfrac{\theta}{2 \pi} = \dfrac{1}{5}$ $\theta = \dfrac{1}{5} \times 2 \pi$ $\theta = \dfrac{2}{5}\pi$ radians